Migration occurs wherever there are birds. Migration is the part of the animal’s life that ischaracterized by geographic movements. This is an incredibly diverse behavior, an adaptation that has been shaped by natural selection. Arctic birds migrate, as do tropical birds. We often think of migrationas the seasonal movement of birds during spring and fall to avoid harsh winters. We have all heard the phrase “fly south for the winter.” This is only partly correct. Scientists have extensivelystudied the evolution of migration. Not one theory has been widely accepted. Explanations include and are not limited to glaciers, changing climatic patterns, and extending dispersal distance. Mostbiologistswill agree that the foundations of all explanations must be food. The availability of food is the driving force in the evolution of migration. Food abundance can increasereproductive output,whereas a lack of food leads to death A bird that can find food willhave a longer life span and produce offspring more readily than one that finds less food. |
Just by moving from place to place a birdcan find more food, and thus migration evolved. It just so happens that many warmer climates have an overabundance of food, whereas cold harsh environments do not. Most species of birds can toleratecolder temperatures given that food is available.Scientists have identified three patterns of migration: complete, partial and irruptive. Complete migration occurs when individuals leave the breedingrange during the nonbreeding season. Many North American birds such as warblers, orioles, hummingbirds, and shorebirds exhibit this migration pattern. Complete migrants may travel incredible distances,sometimes more than 15,000 miles per year. The most common type of migration is partial migration. This is characterized by seasonal movements away from a breeding range by some but not all of the...
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